PROJECT SUMMARY Autism spectrum disorder (ASD) is a common neurodevelopmental disorder, affecting social communication in approximately 1 out of 68 children (CDC, 2016). Because dyadic interactions are central to language and social development, the brain basis of these social communication and engagement processes are of interest. In children with ASD or parents with subclinical language and personality features similar to defining symptoms of autism (known as the broader autism phenotype, BAP), social communication may be interrupted. Despite the dynamic, dyadic nature of social communication, most neuroimaging studies examine individuals one at a time, and often study social communication and engagement during artificial experimental situations, such as listening to recorded speech or viewing faces on a screen. Studying dyadic engagement in natural context is important because engaged, reciprocal interactions promote language and social communication development. This study uses cutting-edge ?social EEG? techniques in order to measure responses in two brains simultaneously and assess their synchrony. Toddlers with and without ASD and their mothers both wear EEG caps and their EEG is recorded during naturalistic interactions designed to elicit (a) joint engagement with and without communication, (b) parallel attention (no communication), or (c) no joint engagement. Behavior is microcoded in to these four mutually-exclusive states. Neural synchrony is then compared across each state, after carefully removing and controlling for motion artifacts. This study breaks new ground in 3 ways, by assessing parent-child synchrony (1) in ASD vs. TD groups, (2) in toddlers, and (3) in relation to dyadic social communication and joint engagement. Because ASD is known to begin early in life and arise from differences in the brain, understanding brain differences in dyadic social communication and joint engagement via the assessment of neural synchrony could provide insight into the mechanisms underlying the disorder. It could also lead to a potential objective marker of brain processes useful for diagnosis and assessment of ASD and of intervention response. Specific aims are as follows: Specific Aim 1. Examine the extent to which different levels of observed behavioral engagement are reflected in differences in EEG neural synchrony among typically-developing (TD) toddler-mother dyads. Hypothesis 1: Dyadic neural synchrony (mother-toddler EEG phase coherence and power similarity) will be greater during periods of dyadic social communication and joint engagement than during parallel attention; synchrony will be lowest during periods of no joint engagement. Specific Aim 2. Determine the extent to which neural synchrony in different dyadic engagement states differs in children with ASD and mothers with BAP traits. Hypothesis 2: Dyads with a child with ASD or a parent with BAP traits will have reduced neural synchrony relative to TD dyads while engaged in social communication and joint engagement but will not differ during parallel attention or no engagement states.